It is often desirable to determine the presence or quantity of a particular enzyme within a test sample. In some cases, the mere presence of an enzyme may, for example, indicate the existence of tissue or organ damage. Likewise, abnormal enzyme concentrations may also indicate other conditions, such as a bacterial or viral infection. For instance, proteases (e.g., aspartic proteases) and metallopeptidases are believed to increase the pathogenicity of Candida albicans, a microorganism that may cause candidal vaginitis (“yeast infection”). The presence or concentration of an enzyme in a test sample may also serve as a diagnostic marker for some types of cancers and other conditions. For instance, prostate-specific antigen (PSA) is a well-known marker for prostate cancer. Other examples of diagnostic markers include cathepsin B (cancer), cathepsin G (emphysema, rheumatoid arthritis, inflammation), plasminogen activator (thrombosis, chronic inflammation, cancer), and urokinase (cancer).
One conventional technique for detecting the presence of an enzyme is described in U.S. Pat. No. 6,348,319 to Braach-Maksvytis, et al. Braach-Maksvytis, et al. functions by sensing the digestion of a substrate by the enzyme. For example, FIG. 1 of Braach-Maksvytis, et al. illustrates a device 10 that includes a first zone 11 and a second zone 12. The first zone 11 is provided with polymer beads 13 (carrier) linked to streptavidin 14 (probe) via a peptide linker 15 that is cleavable by a protease 16. Upon addition of the protease 16, the streptavidin 14 is released and passes to the second zone 12, which includes a biosensor membrane 17 that detects the presence of streptavidin through a change in the impedance of the membrane. (Col. 5, II. 25-30). Unfortunately, however, techniques such as described by Braach-Maksvytis, et al., are far too complex and cost prohibitive for certain types of applications, such as those requiring a relatively quick diagnosis by a patient (self-diagnosis or with the aid of medical personnel).
As such, a need currently exists for a simple and inexpensive technique to accurately detect the presence of an enzyme within a test sample.